1. Field of the Invention
This invention relates generally to apparatus for mixing breathing gas for ventilating the lungs of a patient, and more particularly concerns apparatus suitable for use with a piston type ventilator, for delivering gas to the piston chamber of the ventilator at or near atmospheric pressure, without the need for a compressor, and a method for limiting the peak flow of oxygen supplied to the piston chamber of the ventilator.
2. Description of Related Art
Medical ventilators are designed to ventilate a patient""s lungs with breathing gas to assist a patient in breathing when the patient is somehow unable to adequately breath on his own. Some ventilator systems in the art provide the patient with pressure assistance that is instituted when the patient has already begun an inspiratory effort. Such a ventilator provides an increase in pressure of the breathing gas in the patient airway to assist the flow of breathing gas to the patient""s lungs, thus decreasing the work of breathing by the patient. Conventional pressure controlled ventilator systems implement a gas flow control strategy of terminating breathing gas flow when a target pressure is reached, or after a specified delay at this target pressure.
However, such a control strategy can result in over-pressurization of the patient""s lungs, particularly when high pressure gasses are used for blending the breathing gas mixture, due to the response time delay in reducing pressure after a target pressure is sensed, or after a specified elapsed time. When over-pressurization is sustained, the patient""s lungs can be subjected to excessive pressure for a significant portion of a breath cycle. When this occurs, the possibility exists that the patient will be harmed by a higher than desirable pressure in the lungs, since overpressure can, for example, rupture sutures or blood vessels of a patient that has recently undergone thoracic or abdominal surgery. Similarly, frail or infirm patients, such as asthmatic or emphysemic patients, can also be harmed if airway pressure is excessive.
Bellows and piston types of ventilators allow delivery of a predetermined volume of breathing gas at a desired pressure responsive to the initiation of inspiratory efforts by a patient. Piston based ventilators can typically be made to be more compact than bellows based ventilators, but piston ventilators typically blend pressurized air and oxygen in a high pressure blender. The resultant mixture is then drawn by a piston through a valve that reduces the pressure of the mixture. Such systems typically do not permit the use of room air and pressurized oxygen, and can result in some risk of overpressurization in the event of failure of a high pressure gas delivery valve controlling introduction of one of the breathing gas components into the high pressure blender.
For example, one piston based lung ventilator known in the art utilizes a rolling-seal piston of low inertia and low frictional resistance for delivery of breathing gas, which is mixed in the piston chamber. For mixing of the breathing gas in the piston chamber, the piston chamber has an outlet connected to the airway of the patient, and an inlet with a one-way valve allows air to enter the piston chamber during the exhalation phase of the respiratory cycle. This inlet to the piston chamber, controlled by a solenoid valve, allows introduction of a desired gas mixture into the piston chamber. The solenoid valve for introducing the gas mixture is opened during expiration as the piston reciprocates to a baseline position. The oxygen content of the inspired gas can also be enriched by admitting a continuous flow of oxygen into the piston chamber through another inlet.
In another gas blender for a ventilator, a double ended poppet cooperates with two valve seats to simultaneously open and close both valves to maintain a constant flow ratio. However, in both instances the gases mixed are supplied at high pressure. If a valve controlling the introduction of high pressure oxygen or air fails, it is possible that breathing gas can be provided to a patient at an excessive pressure. In addition, with these types of systems for introducing different gases into a gas delivery cylinder of a ventilator, mixing of the gases can be incomplete, sometimes resulting in delivery of a lower concentration of oxygen to a patient than desired.
It would therefore be desirable to provide the components of a breathing gas for mixing, such as oxygen and air, at approximately ambient atmospheric pressure. An advantage of such an arrangement is that air can readily be supplied from the ambient atmosphere without the necessity of providing a compressor equipment for providing pressurized air. It would be desirable to regulate the pressure of a selected gas, such as oxygen, to approximately ambient pressure, for mixing with ambient pressure air. It would further be desirable to limit the pressure of the selected gas to be mixed with ambient pressure air to an acceptable maximum pressure, so that even if a valve for admitting the selected gas at ambient pressure for mixing fails, breathing gas will not be provided at an excessive pressure. It would also be desirable to provide a gas mixing apparatus that would allow a desired sequencing of introduction of the selected gases to be mixed into a gas delivery cylinder of a ventilator. The present invention meets these needs.
Briefly, and in general terms, the present invention provides for a gas mixing apparatus for a ventilator system for providing the components of a breathing gas for mixing at approximately ambient atmospheric pressure, and that regulates the pressure of a selected gas to approximately ambient atmospheric pressure, for mixing with air at ambient atmospheric pressure. The gas mixing apparatus limits the pressure of the selected gas to an acceptable maximum pressure, so that even if a valve for admitting the selected gas for mixing at ambient pressure fails, breathing gas will not be provided at an excessive pressure.
In one preferred embodiment, the gas mixing apparatus comprises a fixed volume piston chamber having a first gas delivery portion of the piston chamber with an inlet for receiving mixed gas and an outlet for delivering mixed gas to the patient airway during an inspiratory portion of a breath cycle. A reciprocating piston is disposed within the piston chamber, movable between an extended position in the first gas delivery portion of the piston chamber and a retracted position in a second portion of the piston chamber. Means are provided for moving the piston between the extended and retracted positions, and in one preferred aspect of the invention, a reservoir can provide a mixing chamber for mixing a selected gas, such as oxygen, with air, while in another preferred embodiment the selected gas is mixed in a mixing chamber of the piston chamber.
In each embodiment, the mixing chamber preferably includes an air inlet open to the atmosphere, and a flow limiting inlet for admitting the selected gas into the mixing chamber. In one preferred aspect of the invention, the flow limiting inlet comprises a regulator and a sonic orifice allowing a maximum mass flow rate of the selected gas to the mixing chamber. A source of the selected gas is provided for supplying a flow of the selected gas to the mixing chamber, and valve means are provided in fluid communication with the source and the mixing chamber for regulating the flow of the selected gas to the mixing chamber. The gas mixing apparatus also preferably includes control means for controlling the valve means to admit the selected gas to the mixing chamber during at least one interval of time during at least a portion of the breath cycle for a total period of time during the breath cycle to obtain a selected proportion of the selected gas in the mixed gas.
In a first presently preferred embodiment, the mixing chamber comprises at least one reservoir connected in fluid communication with the inlet of the piston chamber, and in another preferred embodiment, the mixing chamber can comprise a plurality of reservoirs or reservoir chambers connected in series in fluid communication with the inlet of the piston chamber. In another preferred embodiment, the mixing chamber comprises a gas mixing portion of the piston chamber between the piston and the second side of the piston chamber.
In another preferred aspect of the invention, although air is supplied at ambient atmospheric pressure, the selected gas is supplied at a pressure above atmospheric pressure, and the gas mixing apparatus further includes a demand valve for reducing the pressure of the selected gas supplied to atmospheric pressure. A pressure sensor is preferably provided upstream of the demand valve for sensing a low supply of the selected gas, and a pressure sensor is preferably provided downstream of the demand valve for detecting failure of the demand valve, and generates a failure signal when failure of the demand valve is detected. A solenoid valve upstream of the demand valve responsive to the failure signal of the downstream pressure sensor is provided to close off the supply flow of the selected gas, preventing excessive pressure buildup of the selected gas from reaching the mixing chamber and preventing overpressurization of the patient airway.
The invention also provides for a method of mixing gas for a ventilator system used for supplying mixed breathing gas to a patient airway during an inspiratory portion of a breath cycle. In a presently preferred method according to the invention, a supply flow of a first selected gas is provided for mixing with a second selected gas. The piston is moved to an extended position to deliver a flow of mixed gas from the gas delivery portion of the piston chamber to the patient airway; and is moved to a retracted position to draw mixed gas into the gas delivery portion of the piston chamber. The supply flow of the first selected gas is controlled to admit the first selected gas for mixing during at least one interval of time period during the breath cycle, and for a total period of time during the breath cycle, to obtain a selected proportion of the first selected gas in the mixed gas.
One preferred embodiment of the method of the invention involves admitting mixed gas during retraction of the piston; and another preferred embodiment of the method of the invention involves admitting mixed gas during extension of the piston to deliver mixed gas to the patient airway. In another preferred embodiment of the method of the invention, introduction of the selected gas and air into a gas delivery chamber is sequenced in the mixing cycle, with a pulse of the selected gas followed by a pulse of air, to help insure adequate mixing of the selected gas and air for delivery to a patient.
From the above, it may be seen that the present invention provides an improved method and apparatus for providing mixed breathing gas for a patient being ventilated by a piston type ventilator. These and other aspects and advantages of the invention will become apparent from the following detailed description, and the accompanying drawings, which illustrate by way of example the features of the invention.